why i cant delete photos from i cloud ,my iPhone is IOS 5.0.1???

Welcome to the Apple community.

You can delete photos from photo stream on your device, you are probably just doing it the wrong way. You cannot delete the photo from the photo itself, you need to delete them from the photo stream photo sheet. Once you are viewing the photo sheet, click on the rectangle with the arrow in it, in the top right corner, select the photos you wish to delete and hit the delete button at the bottom of the screen.